344 research outputs found
Coherent and radiative couplings through 2D structured environments
We study coherent and radiative interactions induced among two or more
quantum units, by coupling them to two-dimensional lattices acting as
structured environments. This model can be representative of atoms trapped near
photonic crystal slabs, trapped ions in Coulomb crystals or to surface acoustic
waves on piezoelectric materials, cold atoms on state-dependent optical
lattices, or even circuit QED architectures, to name a few. We compare coherent
and radiative contributions for the isotropic and directional regimes of
emission into the lattice, for infinite and finite lattices, highlighting their
differences and existing pitfalls, e.g. related to long-time or large-lattice
limits. We relate the phenomenon of directionality of emission with
linear-shaped isofrequency manifolds in the dispersion relation, showing a
simple way to disrupt it. For finite lattices, we study further details as the
scaling of resonant number of lattice modes for the isotropic and directional
regimes, and relate this behavior with known van Hove singularities in the
infinite lattice limit. Further we export the understanding of emission
dynamics with the decay of entanglement for two quantum, atomic or bosonic,
units coupled to the 2D lattice. We analyze in some detail completely
subradiant configurations of more than two atoms, which can occur in the finite
lattice scenario, in contrast with the infinite lattice case. Finally we
demonstrate that induced coherent interactions for dark states are zero for the
finite lattice.Comment: 10 page
Completely subradiant multi-atom architectures through 2D photonic crystals
Inspired by recent advances in the manipulation of atoms trapped near 1D
waveguides and pro- posals to use surface acoustic waves on piezoelectric
substrates for the same purpose, we show the potential of two-dimensional
platforms. We exploit the directional emission of atoms near photonic crystal
slabs with square symmetry to build perfect subradiant states of 2 distant
atoms, possible in 2D only for finite lattices with reflecting boundaries. We
also show how to design massively parallel 1D arrays of atoms above a single
crystal, useful for multi-port output of nonclassical light, by ex- ploiting
destructive interference of guided resonance modes due to finite size effects.
Directionality of the emission is shown to be present whenever a linear
iso-frequency manifold is present in the dispersion relation of the crystal.
Multi-atom radiance properties can be obtained from a simple cross-talk
coefficient of a master equation, which we compare with exact atom-crystal
dynamics, showing its predictive power
Anisotropic quantum emitter interactions in two-dimensional photonic-crystal baths
Quantum emitters interacting with two-dimensional photonic-crystal baths
experience strong and anisotropic collective dissipation when they are
spectrally tuned to 2D Van-Hove singularities. In this work, we show how to
turn this dissipation into coherent dipole-dipole interactions with tuneable
range by breaking the lattice degeneracy at the Van-Hove point with a
superlattice geometry. Using a coupled-mode description, we show that the
origin of these interactions stems from the emergence of a qubit-photon bound
state which inherits the anisotropic properties of the original dissipation,
and whose spatial decay can be tuned via the superlattice parameters or the
detuning of the optical transition respect to the band-edges. Within that
picture, we also calculate the emitter induced dynamics in an exact manner,
bounding the parameter regimes where the dynamics lies within a Markovian
description. As an application, we develop a four-qubit entanglement protocol
exploiting the shape of the interactions. Finally, we provide a
proof-of-principle example of a photonic crystal where such interactions can be
obtained.Comment: 12 pages, 8 figure
Multi-ion sensing of dipolar noise sources in ion traps
Trapped-ion quantum platforms are subject to `anomalous' heating due to
interactions with electric-field noise sources of nature not yet completely
known. There is ample experimental evidence that this noise originates at the
surfaces of the trap electrodes, and models assuming fluctuating point-like
dipoles are consistent with observations, but the exact microscopic mechanisms
behind anomalous heating remain undetermined. Here we show how a two-ion probe
displays a transition in its dissipation properties, enabling experimental
access to the mean orientation of the dipoles and the spatial extent of
dipole-dipole correlations. This information can be used to test the validity
of candidate microscopic models, which predict correlation lengths spanning
several orders of mag- nitude. Furthermore, we propose an experiment to measure
these effects with currently-available traps and techniques
Discording power of quantum evolutions
We introduce the discording power of a unitary transformation, which assesses
its capability to produce quantum discord, and analyze in detail the generation
of discord by relevant classes of two-qubit gates. Our measure is based on the
Cartan decomposition of two-qubit unitaries and on evaluating the maximum
discord achievable by a unitary upon acting on classical-classical states at
fixed purity. We found that there exist gates which are perfect discorders for
any value of purity, and that they belong to a class of operators that includes
the $\sqrt{{SWAP}}. Other gates, even those universal for quantum computation,
do not posses the same property: the CNOT, for example, is a perfect discorder
only for states with low or unit purity, but not for intermediate values. The
discording power of a two-qubit unitary also provides a generalization of the
corresponding measure defined for entanglement to any value of the purity.Comment: accepted for publication in Physical Review Letter
Quantum Otto cycle with inner friction: finite-time and disorder effects
The concept of inner friction, by which a quantum heat engine is unable to
follow adiabatically its strokes and thus dissipates useful energy, is
illustrated in an exact physical model where the working substance consists of
an ensemble of misaligned spins interacting with a magnetic field and
performing the Otto cycle. The effect of this static disorder under a
finite-time cycle gives a new perspective of the concept of inner friction
under realistic settings. We investigate the efficiency and power of this
engine and relate its performance to the amount of friction from misalignment
and to the temperature difference between heat baths. Finally we propose an
alternative experimental implementation of the cycle where the spin is encoded
in the degree of polarization of photons.Comment: Published version in the Focus Issue on "Quantum Thermodynamics
Railway deformation detected by DInSAR over active sinkholes in the Ebro Valley evaporite karst, Spain
Subsidence was measured for the first time on railway tracks in the central sector of Ebro Valley (NE Spain) using Differential Synthetic Aperture Radar Interferometry (DInSAR) techniques. This area is affected by evaporite karst and the analysed railway corridors traverse active sinkholes that produce deformations in these infrastructures. One of the railway tracks affected by slight settlements is the Madrid-Barcelona high-speed line, a form of transport infrastructure highly vulnerable to ground deformation processes. Our analysis based on DInSAR measurements and geomorphological surveys indicates that this line shows dissolution-induced subsidence and compaction of anthropogenic deposits (infills and embankments). Significant sinkhole-related subsidence was also measured by DInSAR techniques on the Castejón-Zaragoza conventional railway line. This study demonstrates that DInSAR velocity maps, coupled with detailed geomorphological surveys, may help in the identification of the railway track sections that are affected by active subsidence
Orthogonal measurements are {\it almost} sufficient for quantum discord of two qubits
The common use in literature of orthogonal measurements in obtaining quantum
discord for two-qubit states is discussed and compared with more general
measurements. We prove the optimality of orthogonal measurements for rank 2
states. While for rank 3 and 4 mixed states they are not optimal, we present
strong numerical evidence showing that they give the correct quantum discord up
to minimal corrections. Based on the connection, through purification with an
ancilla, between discord and entanglement of formation (EoF), we give a tight
upper bound for the EoF of a mixed state of rank 2, given by an
optimal decomposition of 2 elements. We also provide an alternative way to
compute the quantum discord for two qubits based on the Bloch vectors of the
state.Comment: EPL 96, 40005 (2011
Array of planar Penning traps as a nuclear magnetic resonance molecule for quantum computation
An array of planar Penning traps, holding single electrons, can realize an artificial molecule suitable for NMR-like quantum-information processing. The effective spin-spin coupling is accomplished by applying a magnetic field gradient, combined to the Coulomb interaction acting between the charged particles. The system lends itself to scalability, since the same substrate can easily accommodate an arbitrary number of traps. Moreover, the coupling strength is tunable and under experimental control. Our theoretical predictions take into account a realistic setting, within the reach of current technology
- …